Methods of making paper tissue, towel, and the like are well known, including various features such as Yankee drying, throughdrying, fabric creping, dry creping, wet creping and so forth. Conventional wet pressing/dry creping processes (CWP) have certain advantages over conventional through-air drying processes including: (1) lower energy costs associated with the mechanical removal of water rather than transpiration drying with hot air; and (2) higher production speeds which are more readily achieved with processes which utilize wet pressing to form a web. On the other hand, through-air drying processing has been widely adopted for new capital investment, particularly for the production of soft, bulky, premium quality tissue and towel products.
Throughdried, creped products and processes (TAD) products and processes) are disclosed in the following patents: U.S. Pat. No. 3,994,771 to Morgan, Jr. et al.; U.S. Pat. No. 4,102,737 to Morton; and U.S. Pat. No. 4,529,480 to Trokhan. The processes described in these patents comprise, very generally, forming a web on a foraminous support, thermally pre-drying the web, applying the web to a Yankee dryer with a nip defined, in part, by an impression fabric, and creping the product from the Yankee dryer. A relatively permeable web is typically required, making it difficult to employ recycle furnish at levels which may be desired. Transfer to the Yankee typically takes place at web consistencies of from about 60% to about 70%. See also, U.S. Pat. No. 6,187,137 to Druecke et al. which includes disclosure of peeling a web from a Yankee dryer. As noted in the above, throughdried products tend to exhibit enhanced bulk and softness; however, thermal dewatering with hot air tends to be energy intensive. Wet-press/dry crepe operations wherein the webs are mechanically dewatered are preferable from an energy perspective and are more readily applied to furnishes containing recycle fiber which tends to form webs with less uniform permeability than virgin fiber. Moreover, line speeds tend to be higher with wet-press operations.
A wet web may also be dried or initially dewatered by thermal means by way of impingement air drying. Suitable rotary impingement air drying equipment is described in U.S. Pat. No. 6,432,267 to Watson and U.S. Pat. No. 6,447,640 to Watson et al.
Fabric creping has been employed in connection with papermaking processes which include mechanical or compactive dewatering of the paper web as a means to influence product properties. See U.S. Pat. Nos. 4,689,119 and 4,551,199 of Weldon; 4,849,054 and 4,834,838 of Klowak; and 6,287,426 of Edwards et al. Operation of fabric creping processes has been hampered by the difficulty of effectively transferring a web of high or intermediate consistency to a dryer. Note also U.S. Pat. No. 6,350,349 to Hermans et al. which discloses wet transfer of a web from a rotating transfer surface to a fabric. Further United States patents relating to fabric creping more generally include the following: U.S. Pat. Nos. 4,834,838; 4,482,429; 4,445,638 as well as U.S. Pat. No. 4,440,597 to Wells et al. Newer and preferred aspects of processes including fabric-creping are described in the following applications: U.S. application Ser. No. 10/679,862 (Publication No. US-2004-0238135), entitled “Fabric Crepe Process for Making Absorbent Sheet” which application, incorporated herein by reference, discloses particular papermachine details as well as creping techniques, equipment and properties; U.S. application Ser. No. 11/108,375 (Publication No. US 2005-0217814), entitled “Fabric Crepe/Draw Process for Producing Absorbent Sheet” also incorporated herein by reference, provides still further processing and composition information; U.S. application Ser. No. 11/108,458 (Publication No. US 2005-0241787), entitled “Fabric Crepe and In Fabric Drying Process for Producing Absorbent Sheet” and U.S. application Ser. No. 11/104,014 (Publication No. US 2005-0241786), entitled “Wet-Pressed Tissue and Towel Products With Elevated CD Stretch and Low Tensile Ratios Made With a High Solids Fabric Crepe Process” both of which are incorporated herein by reference, provide some further variation as to selection of components and processing techniques. Another application, U.S. Ser. No. 11/451,111, filed Jun. 12, 2006, entitled “Fabric Creped Sheet for Dispensers” incorporated herein by reference, provides information on suitable drying and other manufacturing techniques.
Papermaking processes utilizing creping adhesive, utilizing one or more of the technologies referred to above, are thus well known in the art. It is well-known, for example, that a portion of the bulk of a tissue paper web made by way of conventional wet pressing is usually imparted by creping wherein creping adhesive plays an important role. The level of adhesion of the papermaking web to a dryer cylinder is also of importance as it relates to transfer of the web to a drying cylinder as well as control of the web in between the dryer and the reel upon which a roll of the paper is being formed. Webs which are insufficiently adhered may blister or, even worse, become disengaged from a drying cylinder and cause a hood fire. Moreover, insufficient wet-tack may lead to a transfer failure wherein the web fails to transfer to a drying cylinder and remains imbedded in a fabric causing shutdowns and waste of material and energy. Further, the level of adhesion of the papermaking web to the dryer is of importance as it relates to the drying of the web. Higher levels of adhesion reduce the impedance to heat transfer and cause the web to dry faster, enabling more energy efficient, higher speed operation; provided excessive build-up of adhesive is avoided. Note, however that some build-up is desirable inasmuch as adhesion of the sheet to the dryer occurs largely by means of creping adhesive deposited in previous passes.
Thickness of a coating layer on a Yankee drying cylinder typically increases with time, insulating a wet web from the Yankee surface. In other words, the adhesive coating build-up on the Yankee reduces heat transfer from the Yankee surface. To maintain the same moisture level in the finished product, the Yankee hood temperature is increased accordingly. After two to three hours the hood temperature reaches its upper ceiling and the coating layer needs to be stripped off to reduce the hood temperature to a normal operating window. A new cleaning doctor is typically used to strip off the old coating build-up.
Stripping of the coating, however, results in sheet transfer problems at the pressure roll due to blistering and edge floating.
The problems are more severe when the basis weight of the sheet is reduced. To achieve texturing with lower basis weight, a molding box may be set to a maximum level that results in lower contact areas between the sheet and the Yankee surface when the web is applied to the Yankee surface. Consequently, the sheet develops less adhesion with the Yankee at a constant level of coating application. In addition to this issue, the heat transfer of the Yankee improves significantly immediately after the new cleaning doctor strips off excess coating. This results in a very hot surface and sheet blistering is more severe as the moisture is evaporated. An approach to achieve base sheet caliper with lower basis weight is to reduce wet pressing pressure, which results in a wetter web entering the pressure roll nip and reduces durability of the coating. Consequently less Yankee adhesion and more sheet blistering issues occur.
The present invention provides an improved method to control adhesive build-up which includes intermittently increasing the amount of adhesive supplied to a drying cylinder concurrently with stripping excess adhesive build-up.